European Published Patent Application No. 0 477 400 describes a hydraulic coupler for a piezoelectric actuator in which the actuator transmits a lifting force to a master piston. The master piston is in force-locking connection to a guide cylinder for a slave piston. The slave piston, the guide cylinder and the master piston sealing the guide cylinder form an hydraulic chamber. A spring, which presses the master piston and the slave piston apart, is arranged in the hydraulic chamber. Surrounding an end section of the guide cylinder and the slave piston is a rubber sleeve by which a supply chamber for a viscous hydraulic fluid is sealed from a fuel chamber. The viscosity of the hydraulic fluid is adapted to the ring gap between the slave piston and the guide cylinder.
The slave piston may mechanically transmit a lifting movement to a valve needle, for instance. When the actuator transmits a lifting movement to the master piston and the guide cylinder, this lifting movement is transmitted to the slave piston by the pressure of the hydraulic fluid in the hydraulic chamber, because the hydraulic fluid in the hydraulic chamber is not compressible and during the short duration of a lift only a small portion of the hydraulic fluid is able to escape through the ring gap into the storage chamber formed by the rubber sleeve. In the rest phase, when the actuator does not exert any compressive force on the master piston, the spring pushes the slave piston out of the guide cylinder and, due to the generated vacuum pressure, the hydraulic fluid enters and refills the hydraulic chamber via the ring gap. In this manner, the coupler automatically adapts to linear deformations and pressure-related expansions of a fuel injector.
A disadvantage of conventional hydraulic couplers is that the sealing by a rubber sleeve, which usually is pressed against the end section of the guide cylinder and against the slave piston by two clamping rings, is not fully ensured in the long term. The highly viscous hydraulic fluid and the fuel may mix and the coupler break down. When fuel, such as gasoline, reaches the interior of the coupler, a loss of function may occur since this fluid, due to the low viscosity of the gasoline, may flow too rapidly through the ring gap and no pressure is able to be generated in the pressure chamber during the lift duration.